Update follow value on empty response

[Dantese]

When the API returns no alerts then the returned follow value is ignored. In some scenarios, the follow value might be upgraded without returning any data so we should always check if the follow value has changed and save it. Currently, we stop processing the API response too early:

nfr/alerts/poller.go

Lines 85 to 87 in 86bbd14

	if len(alerts.Alerts) == 0 {
	continue
	}